In the microelectronics field, it is often necessary to be able to detect the onset of a power failure of the main supply to a circuit, such as a microprocessor, and to immediately provide a control or trigger signal to initiate a switchover to a backup or auxiliary power source. To provide fast reaction to such failure onset while minimizing false alarms, it is desirable to have a comparator circuit which compares accurately the slowly varying or D.C. voltage level of the main supply (serving as the comparator input signal level) against the level of the backup or auxiliary power source (also used to power the comparator circuit). Because of circuit nonlinearity, comparison accuracy tends to be limited for comparisons occurring near the supply voltage level. To minimize the effects of circuit nonlinearity and achieve comparison accuracies on the order of 0.1 to 0.2 volts, it is beneficial to employ signal amplification and particularly advantageous in producing a binary output signal to incorporate this amplification as part of a sequentially executed regenerative toggling of a bistable element. The binary output signal so produced can then be used to actuate other circuitry necessary to carry out the switchover to the backup or auxiliary power source.